Transporting stacks of objects, such as orders being assembled in a mail-order operation, presents a particular problem. The stack must be moved very gently from station to station to avoid upsetting it. Furthermore the stack can vary widely in length and overall size, so accurately positioning it and determining its position can be fairly difficult. In particular in the shipping industry where it is necessary to move orders along a belt past various stations where they are filled, the articles added to each pile can be of various sizes.
Pallets or containers make handling the piles easy, but have several drawbacks. First of all they have to be bigger than even the biggest article being moved, so that they usually take up much more space than they normally need. Furthermore they must be recycled back upstream, requiring that a separate conveyor or some other means be provided for doing this. Finally they add to the weight of the items being produced, so the conveyor must be built move this extra mass which is not really part of the product.
Most conveyor systems are too rough for reliable handling of stacks of articles. Gravity-type skate-roller conveyors, for instance, can be counted on to dump a pile of articles as it comes to a rapid halt against an abutment. Normal stop-and-go conveyors, whether of the belt or walking-beam type, subject the stacks to excessive acceleration and deceleration and therefore can also be counted on to upset any stacks being transported.